Portable data carriers (i.e., smart cards or chip cards) are known to include a plastic substrate within which a semiconductor device (i.e., integrated circuit--IC) is disposed for processing digital data. This digital data may constitute program instructions, user information, or any combination thereof. Moreover, these devices are known to be operational in a contacted mode, whereby an array of contact points disposed on the plastic substrate and interconnected with the semiconductor device is used to exchange electrical signals between the portable data carrier and an external card reader, or data communications terminal. Similarly, there exist smart cards that operate in a contactless mode, whereby a radio frequency (RF) receiving circuit is employed to exchange data between the card and a card terminal. That is, the card need not come in physical contact with the card terminal in order to exchange data therewith, but rather must simply be placed within a predetermined range of the terminal. Additionally, there exist smart cards that are alternatively operational in either a contacted mode or a contactless mode. Such cards are equipped with both RF receiving circuitry (for contactless operations) as well as an array of contact pads (for contacted operations), and are commonly referred to as dual mode smart cards.
Whether operating in the contacted or contactless mode, several problems plague the smart card designer. One such problem involves the energy fluctuations created by the integrated circuit on the smart card. These energy fluctuations, which can be caused by common switching noise from a digital signal processor or by current spikes reflective of processing activity, create two somewhat distinct problems during normal smart card operation; namely, receiver sensitivity to the switching noise and security breaches, as next described.
The problem of switching noise is most notable during contactless operation, whereby sensitive analog circuitry shares a common supply rail with the signal processing unit. Referring to FIG. 1, a smart card arrangement 100 includes a substrate 102 for housing the smart card circuitry. The power node 104 is used to supply power, via supply lines 106 and 108 (V.sub.DD and V.sub.SS, respectively), to an optional analog circuit 110 and a signal processor 112. It should be noted that in contacted operation, the analog circuit is not required, as the signal processor 112 receives power directly from an external data communications terminal (not shown). However, in contactless operation, the analog circuit 110 is present, which may include sensitive circuitry whose performance degrades in response to switching noise generated by the signal processor 112. In particular, analog circuit 110 may be a data recovery circuit and required to recover a data signal from a power signal that is modulated with 10% amplitude shift keying (ASK). If the switching noise generated by the signal processor 112 is allowed to couple to the ASK modulated power signal, the data signal may become corrupted. Thus, the problem of switching noise must be addressed in order to improve performance during contactless operations.
Another problem, which exists in both contacted and contactless modes of operation, stems from the digital signature produced by the signal processor 112, wherein each data transfer and instruction execution will typically draw a different amount of energy (e.g., current). By monitoring the input power fluctuations associated with these events, sequences of instruction executions and data transfers can be determined, thereby increasing the likelihood of a security breach. For example, it would be a fairly straightforward, albeit arduous, task to extract encryption keys by monitoring the data transfers performed by the signal processor 112. Thus, the energy fluctuations present during normal operation, in either contacted or contactless mode, can be unscrupulously monitored, leading to an undesirable vulnerability to security breaches.
It is noted that the foregoing problems exist substantially in either the contacted or contactless mode. FIG. 2 shows a more detailed view of the power node shown in FIG. 1, whereby the different modes of power extraction are highlighted. In particular, an impedance network 104-1, which is typically either a magnetic/inductive coil or an electrostatic/capacitive circuit, can be used in the contactless mode to generate the supply rails 106, 108. It should be noted that this arrangement generally complies with ISO standard 14443. Similarly, terminal pads 104-2 constitute the contacted facilities by which the supply rails 106, 108 are supplied. It is noted that these pads, as well as the other pads shown (201-203, 205-207) correspond with the ISO standard 7816. It is further noted that the arrangements 104-1 and 104-2 can be present in isolation on the portable data device, or used in combination for the dual-mode smart card. It is through these mechanisms that security breaches can be undesirably facilitated.
U.S. Pat. No. 5,563,779, entitled "Method And Apparatus For A Regulated Supply On An Integrated Circuit" attempts to solve the problem of digital switching noise recited herein. This approach senses output voltage levels from a circuit and changes the value of a variable capacitor, which in turn modifies the supply voltage and corrects for the changing output level. Regretfully, the circuits used in the above approach do not respond quickly enough to digitally created switching noise, and are thus ineffective on a high-speed, mixed-mode integrated circuit such as those required in today's portable data devices.
Accordingly, there exists a need for an apparatus and method for reducing the deleterious effects of switching noise created by a signal processor on a smart card. In particular, an approach that was usable in a high-speed, mixed-mode integrated circuit would be an improvement over the prior art. Moreover, any device or method that further yielded enhanced security by virtue of reduced energy fluctuations during normal operations would provide a greater advantage over the prior art.